This invention relates to an improved composite board of the type made from cellulose material and a synthetic resin.
A composite board comprises cellulose material such as timber chips, fibres or particles bound together with an adhesive binder. Generally, composite boards are formed by first forming a mixture of the cellulose material and the resin and thereafter consolidating the mixture under pressure, usually in a press or mould, and during the course of compression, or subsequently, curing the resin by the application of heat.
Examples of composite boards include fibre boards where the cellulose material comprises wood fibres, chip boards where wooden chips are utilized and particle boards where the cellulose material is constituted by wood flakes and particles. In each of these board types, the cellulose material may be of varying sizes throughout the board. The present invention is applicable in the formation of any such board but is particularly suitable for the manufacture of particle boards and fibre boards.
The physical strength, resistance to moisture uptake and other physical characteristics of any particular board can be modified by utilizing different resin systems, utilizing more or less resin, modifying the density of the board and/or by utilizing other additives amongst other factors known in the art of board making.
The density of any particular board can be readily modified by changing processing conditions. These conditions include the amount and type of resin and cellulose fibre used, and the pressure under which the composite mixture is compressed whilst the resin is being cured. Standard particle boards currently manufactured range in density from between about 640 to 780 kg/m.sup.3.
Many composite boards are manufactured using binders or resins which are heat setting. The strength and other physical characteristics of any particular board are influenced by the extent to which the resin is cured during the manufacturing process. The primary means for curing the resin in such boards is by the application of heat whilst the board is being compressed. Standard board presses utilize wide heated platens which transfer heat to the board during compression. Whilst this heat is effective in curing the resin towards the surface of the board, it is less effective in curing the core or centre of the board. The evolution of steam is the main heat transfer medium to the core, and this temperature does not exceed much above 105.degree. C. even when the platen temperature is significantly higher.
Because of this problem, it has previously been proposed to utilize heat setting resins which also cure (even in the absence of heat) in acid conditions. Boards utilizing such resins have previously been made in conjunction with a small amount of a compound which will produce an acid during the formation process. For example, ammonium chloride has previously been used within the core of such composites. The ammonium chloride is believed to react with the free formaldehyde that is present in the resin to form hydrochloric acid and the hydrochloric acid itself drives the reaction which cures the resin. This enables one to manufacture a board where the resin in the core is to a large extent cured, notwithstanding the difficulty in heating the core to temperatures high enough to effect heat setting of the resin in this region.
Whilst the use of materials like ammonium chloride improve the core strength of the board produced, the applicants have found that in such boards the core is still weaker than the surface where the resin has been more effectively heat cured.